1. Field of the Invention
The invention relates to a process for the production of a semiconductor device, and more particularly, to a process for an intrinsic gettering heat treatment (hereinafter referred to as "IG heat-treating method") of a silicon crystal substrate.
2. Description of the Related Art
Silicon crystals used in semiconductor devices contain impurity oxygen in a supersoluble amount, and when these crystals are subjected to a heat treatment, the impurity oxygen is precipitated as an oxide of silicon. It is well known that the defect due to this precipitate causes a gettering of metal impurities which have mingled with the crystal, and this gettering action is utilized in the actual production of semiconductor devices. When this precipitate occurs in the zone of element, however, it impairs the element characteristics.
Therefore, it is important to establish an IG heat-treating method capable of controlling, with a high accuracy, the position and speed of the precipitation of the impurity oxygen in a silicon crystal substrate.
It is known that the speed and amount of the precipitate of the impurity oxygen in the silicon crystal are augmented by the presence of impurity carbon in the crystal, and a utilization of this action of the impurity carbon ought to facilitate the precipitation of the impurity oxygen in the silicon crystal having a low oxygen concentration.
Nevertheless, since the mechanism of the precipitation of the impurity oxygen has not been elucidated, the effect of the impurity carbon cannot be accurately incorporated in the conditions for the IG heat treatment, and thus no method has yet been established for the IG heat treatment of the silicon crystal containing impurity carbon.
FIG. 5 illustrates the conventional process for the IG heat treatment. First, a heat treatment is performed at a temperature exceeding 1,000.degree. C., specifically at 1,100.degree. C., for example, for a period of 1.5 hours to form a denuded zone, then a heat treatment is performed at a temperature of from 650.degree. to 800.degree. C., specifically at 700.degree. C., for example, for a period of four hours, to form a seed for the precipitation of oxygen, and thereafter a treatment for the elevation of the temperature to 1,100.degree. C. is performed to induce growth of the seed for oxygen precipitation.
When a silicon crystal containing impurity carbon is subjected to a heat treatment, the impurity oxygen (Oi) approaches the impurity carbon (Cs) and forms a C--O complex defect, which constitutes itself a seed for the precipitation of oxygen. The present inventors have found that when the heat treatment is performed at a low temperature, the C--O complex defect increases. The experiment which has led to this knowledge will be described in detail below.
FIG. 3 is a graph obtained by performing a heat treatment at a varying temperature of from 450.degree. C. to 800.degree. C. on a silicon crystal having an impurity oxygen (Oi) concentration of 15 ppm and an impurity carbon (Cs) concentration of 6 ppm and plotting the results of the heat treatment as to the duration of heat treatment relative to the concentration of C--O complex consequently formed in the silicon crystal, with the temperature of heat treatment as a parameter. The C--O complex concentration is increased by the heat treatment performed at a temperature not exceeding 600.degree. C. and it is conversely decreased and rapidly brought to an equilibrium state by the heat treatment performed at a temperature exceeding 600.degree. C. It has been confirmed that the C--O complex concentration in the equilibrium state is governed by the impurity oxygen (Oi) concentration and the impurity carbon (Cs) concentration in the silicon crystal and the temperature of the heat treatment.
Thus, it has been established that, when the heat treatment is carried out at the temperature conventionally used for this heat treatment, the concentration of the C--O complex serving as the seed for oxygen precipitation is decreased and an ample oxygen precipitation is not obtained.
On the other hand, in the production of semiconductor devices using silicon single crystals, more often than not such silicon crystals produced by the CZ (Czokralski) process under conditions calculated for the concentration [Oi] of impurity oxygen (Oi) to exceed about 28 ppm are used. In these silicon crystals, the concentration [Cs] of impurity carbon (Cs) is generally controlled to the lowest possible level, specifically below 0.5 ppm (see, for example, Japanese Unexamined Patent Publication No. 61-15335). In the classification of crystals by the concentration [Oi] of impurity oxygen (Oi) and the concentration [Cs] of impurity carbon (Cs), these silicon crystals belong to the region indicated as (a) in FIG. 10. These silicon crystals of high-oxygen and low-carbon concentrations are selectively used for the following reason.
(1) First, to utilize the intrinsic gettering effect due to precipitation of oxygen, these crystals must possess an oxidation precipitation tendency exceeding a certain degree. The impurity carbon (Cs) has an action of promoting precipitation of oxygen. When the concentration [Cs] of the impurity carbon (Cs) is below a certain fixed value (0.5 ppm), the concentration [Oi] of the impurity oxygen (Oi) must exceed a certain fixed value (about 28 ppm) to obtain a sufficient amount of oxygen precipitation. In other words, no sufficient oxygen precipitation is obtained with the crystals of low-oxygen and low-carbon concentrations falling in the region indicated as (d) in FIG. 10.
(2) The concentration [Cs] of the impurity carbon (Cs) must be controlled to a fixed value because the oxygen precipitation in a high-oxygen crystal, whose concentration [Oi] of the impurity oxygen (Oi) exceeds 28 ppm, is extremely sensitive to the concentration [Cs] of the impurity carbon (Cs). By the standard of the current techniques, however, a regulation of this fixed value to the lowest possible level (below 0.5 ppm) is the simplest and economically advantageous approach. To be specific, the silicon crystals belonging to the region indicated as (a) in FIG. 10 are easier to manufacture and economically more advantageous than the silicon crystals belonging to the region indicated as (c).
(3) Since the impurity carbon (Cs) is effective in promoting oxygen precipitation, the use of crystals having low-oxygen and high-carbon concentrations belonging to the region indicated as (b) in FIG. 10 raises no problem exclusively from the viewpoint of oxygen precipitation. When these crystals are used, however, the effect of carbon on the oxygen precipitation, namely the amount of carbon-containing oxygen precipitation seed defect, must be controlled. Heretofore, this problem has been evaded as an unusually complicated matter. Where an accurate control of precipitation is required, therefore, these crystals of low-oxygen and high-carbon concentrations have not been selected for use.
The fact that the crystals of high-oxygen and low-carbon concentration [belonging to the region (a) in FIG. 10] have been disseminated more than the crystals of low-oxygen and high-carbon concentrations [belonging to the region (b) in FIG. 10], as reviewed on the basis of the levels of current techniques and studies, may be ascribed rather to the circumstances of the technical development to date than to the essential difference between them.
The conventional technique using crystals of high-oxygen and low-carbon concentrations has the disadvantage that, even when one and the same heat treatment for oxygen precipitation is performed on silicon crystals containing impurity oxygen (Oi) in one and the same concentration, the amount of precipitation is prone to dispersion. In the case of crystals having a high-oxygen concentration, a sufficient oxygen precipitation takes place even in the absence of carbon. The precipitation of this kind (non-carbon precipitation) is governed in a large measure by the amount of the seed for non-carbon precipitation. Since the silicon crystals neither always assume a fixed amount of thermal hystersis during the course of their production nor produce a fixed amount of seed for non-carbon precipitation, the amount of oxygen precipitation is apt to dispersion.
At present, since the seed for non-carbon precipitation defies direct observation, there is no way of estimating the density of the seed for non-carbon precipitation in a given silicon crystal in advance of the step of heat treatment for oxygen precipitation. It is, therefore, difficult to obtain ample repression of the dispersion in the amount of oxygen precipitation in crystals having high-oxygen and low-carbon concentrations. Note, this dispersion occurs most frequently in crystals having the concentration [Oi] of the impurity oxygen (Oi) of from 28 to 34 ppm, which are used prevalently for devices.
Further, the conventional methods of heat treatment of a silicon crystal as previously mentioned have a disadvantage in that an accurate control of the amount of oxygen precipitation in the silicon crystal is difficult because the amount of oxygen precipitate produced in the silicon crystal is dispersed when the thermal hysteresis exerted on the silicon crystal is varied. This drawback has caused variations in the gettering ability among the individual substrates.